Closing a trade by "claiming" the position means you deposit in a fiat or crypto amount equivalent to your loan to buy or sell off the trade. No trading fee is involved since no order is generated.
How to claim a position on Liquid
The Claim button is by default on the far right of the Positions panel. There's a bit of scrolling to do. Click the button once you found it.
If you claim positions regularly, it might help to move the Claim column to the left side.
Click Confirm when prompted, or Dismiss to go back.